During the process of starting uphill in μ split situations, i.e. when differing coefficients of friction of the underlying surface are present at the wheels at the left-hand and right-hand sides of the vehicle, critical situations with respect to the stability of the vehicle can occur depending on how the drive torque engages at the wheels. In particular, the capability of the vehicle to move forward in the desired direction can be adversely affected.
Most motor vehicles which are currently commercially available have an open differential and are equipped with an ESP (Electronic Stability Control system), with a traction control system (ETC—Engine Traction Control) and with a brake traction control system which simulates a locking differential using brake pressure on the slipping wheel. Vehicles with electronically controllable locking differentials are now also available.
An imprecise locking torque can easily bring about an undesired movement of the vehicle irrespective of the locking device on the drive axle. It is necessary to take into account the fact that in the case of a vehicle start uphill in a μ split situation, in particular with a manual transmission, it is very easy to apply an excessively large torque to the drive train, in particular if only part of the drive torque can be converted into propulsion by lack of grip.
The excess torque causes either one of the wheels or both of the wheels of the respective axis to spin, as a result of which the lateral and longitudinal forces which are available as a result of the spinning wheels or the spinning wheel are greatly reduced. If the wheel with the high coefficient of friction of the underlying surface on the drive axle starts to spin, the lateral stability of the axle is adversely affected and the vehicle begins to carry out a yawing movement as a result of gravity and the non-uniform drive forces acting at the driven wheels. These effects can give rise to the following situations. If an excessively low locking torque is applied to the electronically controllable locking differential there is a risk of the motor vehicle rolling back downhill, since sufficient drive torque has not been shifted from the driven wheel with the low coefficient of friction of the underlying surface to the driven wheel with the high coefficient of friction of the underlying surface. The application of an excessively high locking torque to the electronically controllable locking differential can have the following effects: if the drive axle points uphill, that is to say for example a motor vehicle with front wheel drive drives uphill, the vehicle begins a yawing movement about the lower axle, for example the rear axle, similarly to an inverse pendulum. Once this movement has begun, it is difficult to stop it, and the vehicle will become positioned transversely with respect to the underlying surface, or in the worst case slip downhill in a rotating movement. This situation is impossible to control. In the event of the axle which is not the driven axle pointing uphill, for example in the case of a motor vehicle with front wheel drive which drives rearward uphill, the motor vehicle begins a yawing movement about the higher axle, similarly to a normal pendulum. Furthermore, there is a risk of rolling back downhill if the longitudinal force of the driven wheels is reduced by the excess spinning.
The abovementioned examples show that it is significant to apply the correct level of locking torque to the electronically controllable locking differential and to do this at the correct time. Furthermore, it is crucial to regulate the functional conditions of the driven wheel with the high coefficient of friction of the underlying surface carefully, since excessively large or excessively small locking torques can have negative consequences.
Document DE 10 2006 021 652 A1 describes a device and a method for assisting a starting process from a stationary state of a motor vehicle when a μ split situation is present. In this context it is proposed that when the presence of coefficients of friction of the underlying surface which are different between sides is detected, the brake pressure is reduced in a wheel brake which is assigned to a driven wheel on the side of the underlying surface with the high coefficient of friction of the underlying surface, while in the wheel brake which is assigned to a driven wheel on the side of the vehicle with the low coefficient of friction of the underlying surface a brake pressure is maintained.
DE 10 2015 212 948 A1 describes a method for improving the acceleration behavior of the vehicle. In this context, a braking torque acting on a driven wheel of the vehicle is brought about using a stability control system, in order to reduce a slip of the driven wheel. Furthermore, indices are determined to express that a first driven wheel of the vehicle is on a first side of the underlying surface which permits lower frictional engagement with the first driven wheel than a second side of the underlying surface with a second driven wheel of the vehicle